The invention described herein relates generally to data enhancement systems, and more particularly to systems for enhancing the data from measuring instruments comprising at least one microchannel plate.
Spectroscopic measurements of the energy distributions of aggregations of photons and particles are often made. These measurements frequently employ the use of one or more microchannel plates. A microchannel plate is a parallel array of individual electron multiplier channels. Channel diameters are typically in the range of about 12 to 100 microns. Microchannel plates are usually comprised of glass formed as a polygonal or round disk about 2 to 5 centimeters in diameter and about 0.6 to 4 millimeters thick. In their manufacture, glass is first drawn into tubular fibers. Then, lengths of the fiber are formed into a parallel bundle which is fused together by the application of pressure and heat. Plates are made by cutting the fused bundle into slices and polishing the faces of each slice. The channel plates are heated in hydrogen to produce a very thin semiconducting surface film on the channel walls, to obtain the necessary electrical conductance and secondary emission properties required for channel electron multiplication. Finally, metal electrodes are applied to both faces of the plate by vacuum deposition. A microchannel plate is operated in a vacuum with different potentials applied to the electrodes to produce an axial electric field through the channels. When radiation in the form of electrons, photons, X-rays, etc. enters the low potential end of a channel and strikes the inner surface with sufficient energy, electrons are emitted from the surface. The emitted electrons collide with the walls repeatedly as they are accelerated toward the output end of the channel by the applied electric field, producing additional secondaries. Ultimately, a very large number of electrons produced by such multiplication are emitted from the high potential end of the channel. The electronic gain of a channel depends on its length to diameter ratio, on the magnitude of the applied potentials, and on the secondary emission characteristics of the semiconducting inner wall surface.
A schematic representation of a known spectroscopic measurement system employing microchannel plates is given in FIG. 1. Emission, such as photons, charged particles or the like, from a continuously emitting source, 10, is collimated by collimator 12 and introduced to a dispersive element, 14, such as a grating, a bent crystal, a magnet or the like, and thereby spatially resolved by energy into a spectral distribution. It is frequently of great theoretical and practical importance to measure these spectral distributions. An often used technique for doing this, as indicated in FIG. 1, is to allow the dispersed spectral distribution to encounter a microchannel plate, 16. The particular location at which any spectral particle hits the microchannel plate is determined by the energy of the particle. This energy of a spectral particle is represented by E.sub.1, E.sub.2, and E.sub.3 in FIG. 1. Typically, when a single spectral particle hits a channel, or pore, of a microchannel plate, a pulse comprising on the order of 1,000 electrons is produced. Often, as shown, two microchannel plates, 16 and 18, are employed in series, thereby together producing a pulse, representing the single spectral particle, comprising on the order of 1,000,000 electrons. These electrons strike a phosphor layer 20 and produce photons which are conducted by a fiber-optics element 22 to a linear diode array 24. Analog output from the linear diode array may be amplified, stored in a memory, or subjected to various forms of electronic logic in unit 26. Ultimately, the combined output from the linear diode array, generated by a huge number of spectral particles, is accumulated and may be displayed on an oscilloscope, 28, in a trace representative of the spectral distribution.
A linear diode array provides N channels of information, 1, 2, . . . N, with n typically varying from 1 to 1024. The numbering of the N channels is consecutive along the linear diode array so as to correspond to a monotonic sequence of spectral particle energies. Each channel n contains an analog signal amplitude A.sub.n. The dynamic range of a linear diode array channel is typically about 250. The value of A.sub.n is zero in those channels that are not activated. Ideally a single spectral particle should cause the activation of only a single linear diode array channel, with that single channel at the location appropriate for the energy of the single spectral particle. Linear diode array channels remain activated for only a limited, very short period of time. As shown in FIG. 1, a serious disadvantage of the prior art is the spatial spreading of the electrons produced by the microchannel plate and linear diode array apparatus. This results in a single spectral particle causing the activation of not one but rather a multiplicity of linear diode array channels. The consequence of this is a spreading out, or blurring, of the output data. This is indicated in FIG. 2 which schematically shows the input to memory, 30, representing one spectral particle. Ideally, only one channel should contain a non-zero amplitude above the discrimination level. When data from a great many spectral particles are summed to give a measured spectral distribution, this blurring problem causes the result to be lacking of detailed features, thereby greatly diminishing the value of the measurement.
This blurring problem could be overcome by finding the centroid channel location corresponding to each individual spectral particle. The centroid channel location for a spectral particle is given mathematically by .SIGMA.nA.sub.n /.SIGMA.A.sub.n. That is, in principle the accumulating data could be separately analyzed for the centroid channel location of each individual spectral particle, with only the centroid channel locations being processed as output. The problem in doing this is that in the usual case data is being accumulated for individual spectral particles at a very high rate, such as thousands per second. Consequently, even though a very limited number of centroid channel location calculations are possible, there is presently no adequate means for calculating centroid channel locations for spectroscopic measurement systems such as the system schematically represented in FIG. 1 which must maintain an extremely high rate of data throughput.
Resistive anode position-sensitive detectors are discussed by Lampton et al., Rev. Sci. Instrum. 50 (9), 1979, pages 1093 to 1097. These detectors encode the location of an event by distributing the charge of the electron cloud produced by a microchannel plate among a number of output terminals. In other words, approximate centroid channel locations are determined by roughly balancing the outputs from the sides of resistive strips. Unfortunately, this technique is very slow and of limited accuracy. Instruments of this type, which tend to be relatively expensive, are supplied by Surface Science Laboratories, Inc. of Palo Alto, Calif.
A microchannel plate used as a single photon detector is described by Parkes et al., in Nuclear Instruments and Methods 121 (1974), pages 151 to 159. An anode collector plate is placed close to the output face of the biased microchannel plate so as to deliver the charge output to a sense amplifier. The detector is made position sensitive by arranging the collector plate as an RC transmission line terminated in suitable amplifiers and filters. Several circuit arrangements permit the extraction of position information. The scheme requires that a very large amount of data be converted from analog form to digital form. Positional data obtained by the detector is spatially spread out in a manner characterized by a line spread function that is approximately Gaussian. For the system as described high rate operation is limited by the pulse pile-up behaviour of the terminal amplifiers.
Coincidence-anode multi-anode microchannel arrays are described by Timothy et al. in SPIE Vol. 265, "Shuttle Pointing of Electro-Optical Experiments", pages 93 to 105. These arrays employ two sets of mutually insulated anode electrodes exposed to the output face of the microchannel plate. In these arrays the spatial location of an event is determined by the simultaneous detection of a charge pulse on the two sets of anode electrodes. Using this technique, a total of a times b pixels can be uniquely defined using only a total of a plus b sets of anode electrodes. This technique is not related to the problem of determining the centroid channel location of a multiplicity of activated linear diode array channels.
Case et al., in U.S. Pat. No. 3,958,079 issued May 18, 1976, teaches a device for improving the vertical resolution of a two-dimensional television-based radiation detection system. Data indicative of the centroid location of an image is obtained by digital logic circuits which determine the location and number of the raster scan lines detecting the image. The centroid value is assumed to depend only on the number of raster lines displaying the image, and no account is taken of any intensity variation over the extent of the image. In other words, no account is taken of the amplitude of the image signal. For example, if an event is imaged over scan lines 10 through 13, the centroid of the image is determined to be located exactly between lines 11 and 12.
Miller, in U.S. Pat. No. 3,591,785 issued July 6, 1971, teaches a signal averaging system for determining the average magnitude of a plurality of separate input signals of diverse magnitudes. An output signal representative of the sum of all the separate input signals is first produced. Then, this summed output signal is divided by the total number of the separate input signals. The average magnitude of a plurality of signals is entirely different from the locational centroid of a plurality of responding data channels.
Willis et al., in U.S. Pat. No. 4,357,673 issued Nov. 2, 1982, teach an instrument which performs a series of sample, reference, and dark measurements to generate a normalized result. The average and the variance of measurements made by the instrument are calculated. This enables the cancellation of measurement variation due to variation in instrument response.
Thus, for spectroscopic measurement systems as schematically indicated in FIG. 1 to produce detailed and precise measured spectral distributions, the major problem of how to rapidly and accurately determine the centroid channel location of the multiplicity of linear diode array channels activated by the spatially spread out pulse of electrons produced by one or more multichannel plates driven by a single spectral particle must be solved.